Both sliding and hinged doors need the facility to be made secure against unauthorised opening. Bolt units have therefore been developed which have fittings for attachment to an external surface of the door, specifically with the bolt being slidable in a bolt housing or casing between guides; in use the bolt unit includes a slidable bolt which can be moved into and out of engagement with a keeper secured to a fixed member, usually to the upright of a fixed outer door frame within which the door panel is hingedly mounted.
A door secured by a bolt unit is often vulnerable to unauthorised opening upon "bursting" of the securement, with the bolt being disengaged from its keeper. Such bursting of the bolt can for instance be effected by a blow impacting on the door edge perpendicular to the plane of the door, and which for a hinged door would be delivered in the door opening direction. A bolt is strong against bending and shear forces, but nevertheless if the bolt is mounted in cantilever the inserted (unsupported) and can often be sprung from its keeper by a determined blow.
It is of course desirable that unlawful opening movement of a door generally parallel to its plane (as might occur by use of a housebreaker's jemmy) also be made more difficult. Improved security against bursting movement perpendicular to the door frame should not result in reduced security against unauthorised opening parallel to the door frame; it is an advantage of embodiments of the invention that increased security can be provided against such attempted door openings parallel to its plane.